Since being excellent in mechanical strengths, such as abrasion resistance, chemical stability, thermal conductivity, heat resistance, and the like, alumina has many application fields and has been widely used in the fields of abrasives, electronic materials, heat dissipation fillers, optical materials, biomaterials, and the like. In particular, for the filler application, there has been demanded alumina with the high degree of α crystallization which has high chemical and physical stability and having an approximately spherical shape which hardly abrades an apparatus or the like. Furthermore, in the application in which a heat dissipation property of alumina is expected, in order to realize high filling of high thermal conductive alumina having a high degree of α crystallization in resin, alumina particles each having an approximately spherical shape have been demanded.
A general and most inexpensive method for manufacturing α-alumina is Bayer's method which uses bauxite as a raw material. According to Bayer's method, aluminum hydroxide (gibbsite) or transition alumina is manufactured from bauxite as a raw material, followed by performing calcination in the air, so that an α-alumina powder is manufactured. However, α-alumina obtained by Bayer's method is aggregates of shapeless particles, and hence, the particle shape and the particle diameter thereof are difficult to control.
Because of the background as described above, attention has been paid to an alumina synthesis which can form α crystal and which can control the particle shape and the particle diameter thereof. For example, there has been disclosed a method for manufacturing hexagonal plate-shaped α-alumina having an average particle diameter of 2 to 20 μm and a well-developed face [001] in which after a fluorine-based flux having a melting point of 800° C. or less is added as a mineralizer to aluminum hydroxide or transition alumina, calcination is performed at a high temperature (for example, see Patent Literature 1). However, by this method, since all the particle shapes are hexagonal plates, the problems in that an excellent abradability and a high filling property in resin are difficult to obtain may arise.
In order to synthesize polyhedral α-alumina particles each having an approximately spherical shape, a plurality of proposals has been made in the past. For example, there has been disclosed a method for manufacturing an α-alumina powder having an average particle diameter of 1 to 10 μm and a ratio (D/H ratio) of approximately 1, the ratio being a ratio of a diameter D orthogonal to the crystallographic C axis to a height H parallel thereto, in which boron and a boron-based compound, each containing ammonium, are used as a mineralizer, and aluminum hydroxide (gibbsite) obtained by Bayer's method is calcined at 1,200° C. or more (for example, see Patent Literature 2). In addition, there has been disclosed a method for manufacturing polyhedral α-alumina single crystal particles having an average particle diameter 0.1 to 30 μm and a D/H ratio in a range of 0.5 to 3 in which by the use of a halogen gas, transition alumina and/or an alumina raw material to be formed into transition alumina by a heat treatment is calcined at 1,100° C. (for example, see Patent Literature 3). Furthermore, there has been disclosed a method for manufacturing polyhedral α-alumina particles having an average particle diameter 0.5 to 6 μm and a D/H ratio in a range of 1 to 3 in which a mixture obtained by addition of a small amount of a fluorine compound or a small amount of a fluorine compound and a boron compound to an alumina raw material is calcined at a high temperature of 1,100° C. or more (for example, see Patent Literature 4). However, by any one of the methods disclosed in those patent literatures, in the manufacturing of polyhedral α-alumina particles, the crystal growth of the face [001] cannot be significantly suppressed, and the formation of particles each having an approximately spherical shape are difficult from theoretical and experimental points of views.
In order to completely suppress the growth of the face [001] of a polyhedral α-alumina crystal, the formation of a hexagonal bipyramidal ruby crystal only having the face [113] has been reported in which by the use of molybdenum oxide (MoO3) as a flux agent, calcination is performed at a high temperature (for example, see Non-Patent Literature 1). According to the above Non-Patent Literature 1, since molybdenum oxide is selectively adsorbed to the face [113] of the ruby crystal, crystal components are not likely to be supplied to the face [113], and as a result, the appearance of the face [001] can be completely suppressed. In addition, Patent Literature 5 has disclosed a method for manufacturing a hexagonal bipyramidal artificial corundum crystal having a particle diameter of 1 to 3 mm in which a mixture of molybdenum oxide, alumina, and another auxiliary agent (95% of molybdenum oxide is contained) is calcined at 1,100° C.
However, by the methods described above, polyhedral α-alumina particles each having an approximately spherical shape and a particle diameter of 100 μm or less, which are to be used as abrasives and resin fillers in various fields, are still difficult to manufacture. In addition, since a large amount of molybdenum oxide is used as a flux agent, the problems of environment and cost may arise in some cases. By the present α-alumina synthetic techniques, α-alumina particles, main component particles of which each have a crystal face other than the face [001] as a main crystal face and a polyhedral shape other than a hexagonal bipyramidal shape, has not been synthesized.